Recently, compact and light surface acoustic wave (SAW) filters have been used as filters in mobile communication devices in terms of downsizing, weight saving and frequency raising. It is now required to reduce the number of parts to achieve further improvements in downsizing and weight saving of the mobile communication devices and to realize a SAW filter having a new function. For example, a balance filter having an unbalance-balance conversion function (balun function) has been proposed. Balanced signals use a pair of signal lines and define data by the potential difference between the signal lines. The signals on the signal lines have an identical amplitude and a 180-degree phase difference. An unbalanced signal defines data by the potential of a single signal line to the ground potential.
Balance filters having the unbalance-balance conversion function are described in Japanese Laid-Open Patent Publication Nos. 2003-528523 (Document 1) and 2003-124777 (Document 2). The balance filter described in Document 1 has a plurality of ground terminals. The balance filter described in Document 2 has six electrode pads on a package. Thus, there is a difficulty in further downsizing of the balance filters. If only one ground terminal is used, the degree of balance will be degraded and the bandpass characteristic will be deteriorated.